Refrigerant Circuit And Method For Operating A Refrigerant Circuit

ABSTRACT

There is provided a refrigerant circuit comprising a compressor ( 10 ), a condenser or gas cooler ( 12 ), an ejector ( 16 ) with a high-pressure connection and a suction connection, a pre-evaporator ( 18 ), a separator ( 20 ) with a liquid phase output and a gas phase output, a low-temperature evaporator ( 28 ) which is arranged between the liquid phase output of the separator ( 20 ) and the suction connection, and a superheating evaporator ( 24 ) which is arranged between the gas phase output of the separator ( 20 ) and the suction side of the compressor ( 10 ). A method for operating a refrigerant circuit provides for expanding condensed or supercritical refrigerant in an ejector ( 16 ), then pre-evaporating it, then separating the predominantly liquid phase from the predominantly gaseous phase, further evaporating the predominantly liquid phase and supplying it to a suction connection of the ejector ( 18 ), and completely evaporating the predominantly gaseous phase before supplying it to a compressor ( 10 ).

The invention relates to a refrigerant circuit as used as part of an air conditioning unit, in particular for a motor vehicle.

In general, there is the desire in such refrigerant circuits to increase the efficiency in order to reduce the amount of energy required in order to operate the refrigerant circuit and thus ultimately to reduce the fuel consumption of the motor vehicle.

The object of the invention is to provide a refrigerant circuit which is characterised by a high degree of efficiency.

In order to achieve this object, there is provided according to the invention a refrigerant circuit comprising a compressor, a condenser or gas cooler, an ejector with a high-pressure connection and a suction connection, a pre-evaporator, a separator with a liquid phase output and a gas phase output, a low-temperature evaporator which is arranged between the liquid phase output of the separator and the suction connection, and a superheating evaporator which is arranged between the gas phase output of the separator and the suction side of the compressor. In order to achieve the object, there is also provided according to the invention a method for operating a refrigerant circuit, in which condensed refrigerant or supercritical gas is expanded in an ejector, then is partially evaporated, then the predominantly liquid phase is separated from the predominantly gaseous phase, the predominantly liquid phase is evaporated in a low-temperature evaporator and is supplied to a suction connection of the ejector, and the predominantly gaseous phase is completely evaporated before being supplied to a compressor. The invention is based on the main concept of evaporating the refrigerant after expansion in three steps. In a first step, approximately one-third of the liquid refrigerant is evaporated to the pressure level at the output of the ejector. Then the separation between the predominantly gaseous phase and the predominantly liquid phase takes place in the separator. The predominantly liquid phase, which once again is approximately one-third of the refrigerant, is evaporated via the low-temperature evaporator and is fed back to the pre-evaporator via the suction connection of the ejector. The predominantly gaseous phase is passed through the superheating evaporator, which is connected to the suction connection of the compressor. In this way, it is ensured that only gaseous refrigerant is supplied to the compressor. Furthermore, since each of the evaporators has a specific task to perform, it can be specially designed for this. This ensures a high degree of efficiency.

Compared to other systems known from the prior art, the solution according to the invention has the advantage that it is cost-effective, since there is no need for electronic control. Furthermore, a system is provided which is characterised by a good ejector effect with a high throughput over the entire operating range. There is no need for a second expansion device, and the throughput through the suction connection of the ejector is not excessively high. There is no need for a pre-throttle.

According to one preferred embodiment of the invention, it is provided that the evaporators are not flowed through directly by the conditioning air that is to be cooled, but rather by a heat transfer medium which flows through a heat exchanger. This embodiment provides an indirect cooling system which, if well designed, has the same degree of efficiency as or even a higher degree of efficiency than a conventional direct cooling system, i.e. a system in which the air to be cooled is passed directly through the evaporators instead of through the separate heat exchanger provided in an indirect system.

A particularly high degree of efficiency can be achieved if the evaporators are counterflow evaporators. In this way, the optimal temperature difference between the heat transfer medium and the refrigerant can be used for each of the different evaporation steps.

Water and/or glycol may be used as the heat transfer medium.

Preferably, the evaporators are designed in such a way that the power of each evaporator lies in the range between 20 and 40% of the total power of all the evaporators. In particular, the evaporating power can be distributed equally, so that each evaporator provides approximately one-third of the evaporating power.

According to one preferred embodiment of the invention, it is provided that the compressor is electrically driven. In this way, the compressor power is independent of the rotational speed of the combustion engine which is otherwise usually used for driving purposes, so that the refrigerant circuit can be better controlled. Furthermore, the electronic control of the compressor makes it possible to use an ejector of simple design with a constant nozzle cross section, since the refrigerant throughput can be suitably controlled.

According to one alternative embodiment, it is provided that the nozzle cross section of the ejector is controllable. This makes it possible to adapt the ejector to very different refrigerant mass flows.

According to one preferred embodiment of the invention, it is provided that the mass flow of the heat transfer medium is controlled in such a way that the temperature difference ΔT_(WT) of the heat transfer medium between the output and the input of the heat exchanger is equal to x times the temperature difference ΔT_(L) of the air between the input and the output of the heat exchanger, wherein x is between 0.7 and 1.3, in particular between 0.9 and 1.1. The indirect cooling system makes it possible, by controlling the mass throughput of the heat transfer medium, to adjust the temperature difference at the heat exchanger so that an optimal efficiency is obtained. In particular, the temperature difference for the air flowing through the heat exchanger and the heat transfer medium flowing through the heat exchanger is set to approximately the same value.

The invention will be described below with reference to a preferred embodiment which is shown in the appended drawings. In the drawings:

FIG. 1 schematically shows a refrigerant circuit according to the invention;

FIG. 2 shows the evaporator region of FIG. 1 on an enlarged scale;

FIG. 3 shows a temperature diagram for the evaporator region; and

FIG. 4 shows an enthalpy diagram for the refrigerant circuit.

FIG. 1 shows a refrigerant circuit 5 which comprises an electrically driven compressor 10, a condenser or gas cooler 12 and an evaporator region 14. The condenser or gas cooler 12 is combined with an internal heat exchanger 13, by means of which heat from the refrigerant on the high-pressure side can be transferred to the low-pressure side. The term “condenser” is used here as an encompassing term for “condenser or gas cooler”.

The evaporator region 14 has an ejector 16, by means of which the refrigerant circulating in the refrigerant circuit can be expanded. On the low-pressure side, the ejector 16 is adjoined by a pre-evaporator 18, the output of which is connected to a separator 20. The separator has a gas phase output 22 which is connected to a superheating evaporator 24. The output of the superheating evaporator 24 leads via the internal heat exchanger 13 to the suction side of the compressor 10. The separator 20 is also provided with a liquid phase output 26, to which a low-temperature evaporator 28 is connected. The output of the low-temperature evaporator 28 is connected to a suction connection 30 of the ejector 16. The separator 20 is also provided with an oil return 32.

Each of the evaporators 18, 24, 28 is connected to a heat exchange circuit 34 which comprises a heat exchanger 36 and a pump 38. As the heat exchange medium in the heat exchange circuit 34, use may be made for example of water and/or glycol. The heat exchanger 36 is preferably designed as a cross-counterflow heat exchanger and is part of an air conditioning unit. The heat exchange medium is passed from the heat exchanger 36 firstly through the superheating evaporator 24, then through the pre-evaporator 18 and then through the low-temperature evaporator 28, before it returns to the heat exchanger 36. All the evaporators are designed here as counterflow evaporators.

During operation of the refrigerant circuit, the refrigerant compressed by the compressor 10 and in the liquid or supercritical state at the output of the condenser or gas cooler 12 is passed through the ejector 16, in which it expands. It then flows through the pre-evaporator 18, in which approximately one-third of the refrigerant mass flow is evaporated. The mixture of liquid and gaseous coolant is then separated in the separator 20 into an essentially gaseous fraction and an essentially liquid fraction. The essentially liquid fraction flows via a throttle to the low-temperature evaporator 28, in which it is (largely) evaporated. The refrigerant is then aspirated by the suction connection 30 of the ejector 16 and is fed back to the pre-evaporator 18. The essentially gaseous fraction of the refrigerant passes from the separator 20 into the superheating evaporator 24, in which the remaining liquid components are evaporated. The refrigerant in vapour form is also superheated. It then passes via the internal heat exchanger 13 to the suction side of the compressor 10.

The quantity of heat required in order to evaporate the refrigerant is supplied via the heat exchange circuit 34. The heat exchange medium, which is at a high temperature level after flowing through the heat exchanger 36, first flows through the superheating evaporator 24. After flowing through the superheating evaporator 24, the heat exchange medium is at a medium temperature level and flows through the pre-evaporator 18. After leaving the pre-evaporator 18, the heat exchange medium is at a low temperature level and is passed through the low-temperature evaporator 28. From there, it passes to the heat exchanger 36, where it draws heat from the air that is to be cooled.

With reference to FIGS. 2 to 4, the heat transfers in the evaporator region 14 and in the heat exchanger 36 will be described below.

The refrigerant has at the point E at the output of the ejector a temperature of approximately 0° C. This temperature remains constant through the pre-evaporator 18. At the point I at the input of the low-temperature evaporator 28, the predominantly liquid phase of the refrigerant has a temperature of −5° C., which it also has at the point J at the output of the low-temperature evaporator 28. The predominantly gaseous phase of the refrigerant has at the point H at the input of the superheating evaporator 24 a temperature of 0° C., while it has a temperature of 10° C. at the point K at the output of the superheating evaporator 24. Said values are examples of a preferred operating state of the refrigerant circuit.

FIG. 3 shows the course of the temperature of the refrigerant in the refrigerant circuit 5, of the heat exchange medium in the heat exchange circuit 34, and of the air L which flows through the heat exchanger 36. It can be seen that the temperature of the heat exchange medium circulating in the heat exchange circuit 34 drops as it flows through the three evaporators 24, 18 and 28. This drop corresponds to the different, rising temperature levels of the refrigerant in the three evaporators 28, 18 and 24. It can be seen that at least a temperature difference of 4 K exists between the temperature of the heat exchange medium and the temperature of the refrigerant. This ensures a good heat transfer. The evaporators 28, 18 and 24 are designed here as counterflow evaporators, so that the temperature difference is maintained across the entire evaporator in each case. For the sake of completeness, it is also possible to see the course of the temperature of the air L which flows through the heat exchanger 36. The latter is designed as a cross-counterflow heat exchanger, so that the temperature difference between the air and the heat exchange medium is kept approximately constant, here at a value of 10 K, while the air is cooled from 25° C. to 5° C. and the heat exchange medium is heated from −3° C. to +16° C.

The described refrigerant circuit can preferably be used in electric vehicles, hybrid vehicles or vehicles which are operated by fuel cells, since these usually comprise an electric compressor and also a battery of sufficient capacity. The refrigerant circuit can therefore also be used for the air conditioning of the vehicle at a standstill or for the air pre-conditioning of a parked vehicle.

The system can be operated with an optimised refrigerant mass throughput in almost all operating states, so that the suction effect at the suction connection of the ejector is great enough to ensure a sufficient refrigerant throughput through the low-temperature evaporator 28.

The refrigerant circuit can be operated with all refrigerants which allow operation according to the Carnot principle, for example R134a or R744.

One particular advantage of the described refrigerant circuit consists in that, on account of the use of the pre-evaporator 18, only a relatively small quantity of refrigerant has to be evaporated in the low-temperature evaporator 28. Due to the lower mass throughput through this evaporator, the pressure ratio of the ejector between the suction pressure at the suction connection of the compressor 10 and the suction connection on the ejector 16 is increased so that, for the same required suction pressure at the low-temperature evaporator 28, the pressure on the suction side of the compressor 10 is higher, which leads to a better coefficient of performance of the refrigerant circuit and thus to a lower fuel consumption.

Due to the advantageous splitting of the evaporator work between three separate evaporators which are in each case flowed through in countercurrent by the heat exchange medium, a sufficient temperature difference between the refrigerant and the heat transfer medium is ensured at any point of the evaporator. The evaporators can therefore be designed to be relatively small. Due to the low temperature level of the low-temperature evaporator 28, the temperature of the heat exchange circuit 34 can be reduced below the saturation temperature of the refrigerant at the suction connection of the compressor 10. In this way, it is ensured that the present indirect system, in which the evaporators do not cool the air directly but rather are flowed through by a heat exchange medium, achieves the necessary temperature drop and thus a high degree of efficiency.

The heat exchanger 36 is designed with regard to an optimal ratio between the pressure drop and heat transfer coefficient for a mass throughput of heat exchange medium of between 70 l/h and at most 300 l/h, preferably between 120 l/h and 250 l/h. The average of all local temperature differences in the heat exchanger 36 is maximal, so that the best heat exchanger performance can be achieved by the same heat exchanger surface area and the same heat exchanger design. The total specific heat capacity in the heat exchange circuit 34 should be in the range between 5 kJ/K and 15 kJ/K.

The minimum rotational speed of the compressor can be increased to such a value that the suction effect of the ejector is satisfactory. Furthermore, the compressor can be operated in a cyclical manner if the required refrigerating power falls below a certain value. The heat capacity of the heat exchange circuit 34 then acts as a cold store during the operating phases in which the compressor is switched off. 

1. A refrigerant circuit comprising a compressor (10), a condenser or gas cooler (12), an ejector (16) with a high-pressure connection and a suction connection, a pre-evaporator (18), a separator (20) with a liquid phase output and a gas phase output, a low-temperature evaporator (28) which is arranged between the liquid phase output of the separator (20) and the suction connection (30) of the ejector (16), and a superheating evaporator (24) which is arranged between the gas phase output of the separator (20) and a suction side of the compressor (10).
 2. A refrigerant circuit according to claim 1, characterised in that the evaporators (18, 24, 28) are flowed through by a heat transfer medium which flows through a heat exchanger (36).
 3. A refrigerant circuit according to claim 2, characterised in that the evaporators (18, 24, 28) are counterflow evaporators (18, 24, 28).
 4. A refrigerant circuit according to claim 2, characterised in that the heat transfer medium is water or a mixture of water and glycol.
 5. A refrigerant circuit according to claim 2, characterised in that the heat exchanger (36) is part of an air conditioning device.
 6. A refrigerant circuit according to claim 2, characterised in that the heat exchanger (36) is a cross-counterflow heat exchanger.
 7. A refrigerant circuit according to claim 1, characterised in that the pre-evaporator (18) is arranged upstream of the separator (20) relative to the flow direction of the refrigerant.
 8. A refrigerant circuit according to claim 1, characterised in that the low-temperature evaporator (28) and/or the superheating evaporator (24) is/are arranged upstream of the separator (20) relative to the flow direction of the refrigerant.
 9. A refrigerant circuit according to claim 1, characterised in that the pre-evaporator (18), the low-temperature evaporator (28) and/or the superheating evaporator (24) form a one-piece evaporating element (14).
 10. A refrigerant circuit according to claim 9, characterised in that the ejector (16) is integrated in the one-piece evaporating element (14).
 11. A refrigerant circuit according to claim 1, characterised in that it has an internal heat exchanger (13), by means of which heat can be transferred from the high-pressure side to the low-pressure side.
 12. A refrigerant circuit according to claim 11, characterised in that the internal heat exchanger (13) is combined in an integrated manner in the condenser or gas cooler (12).
 13. A refrigerant circuit according to claim 1, characterised in that the power of each evaporator (18, 24, 28) lies in the range between 20 and 40% of the total power of all the evaporators.
 14. A refrigerant circuit according to claim 1, characterised in that the compressor (10) is electrically driven.
 15. A refrigerant circuit according to claim 1, characterised in that a nozzle cross section of the ejector (16) is controllable.
 16. A method for operating a refrigerant circuit, in which condensed or supercritical refrigerant is expanded in an ejector (16), then is partially evaporated, then the predominantly liquid phase is separated from the predominantly gaseous phase, the predominantly liquid phase is evaporated in a low-temperature evaporator (28) and is supplied to a suction connection of the ejector (16), and the predominantly gaseous phase is completely evaporated before being supplied to a compressor (10).
 17. A method according to claim 16, characterised in that the predominantly gaseous phase is superheated.
 18. A method according to claim 16, characterised in that the mass flow of the heat transfer medium is controlled in such a way that the temperature difference ΔT_(WT) of the heat transfer medium between the output and the input of a heat exchanger (36) is equal to x times the temperature difference ΔT_(L) of the air between the input and the output of the heat exchanger (36), wherein x is between 0.7 and 1.3.
 19. A method according to claim 18, characterised in that x is between 0.9 and 1.1. 